Project Summary/Abstract: More than 30 years after the discovery of HIV, there are still no vaccines or microbicides to prevent HIV infection. Antiretroviral therapy (ART) is successful in suppressing virus replication, but it does not clear the virus, offers only partial immunologic recovery, and requires lifelong adherence. A better understanding of basic HIV?host cell interactions is needed to develop new strategies to prevent HIV infection. This proposed study seeks to investigate the interplay of HIV with antigen-presenting cells (APCs) and helper CD4 T (Th) cells in the presence of anti-HIV antibodies (Abs) that retard virus spread to Th cells. CD4 Th cells are the main cell type infected by HIV, but not all Th cells are equally vulnerable. As compared with Th1 cells, Th17 cells are preferentially targeted by HIV, in part due to higher expression of HIV envelope (Env) receptors, including ?4?7. Our recent studies showed that Th17 cells are also preferentially targeted by HIV transmitted from APCs. However, different outcomes arise from HIV interactions with Th cells and distinct APCs. HIV-exposed monocytes efficiently transmit virus to Th17 cells while stimulating Th proliferation, resulting in expansion in the number of infected Th17 cells. In contrast, virus transmission from HIV-exposed monocyte-derived dendritic cells (MDDCs) causes the number of Th17 cells to decline. Further, we observed that in the immunological synapses between Th cells and APCs, HIV Env enhances Th cell activation by acting like a co-stimulatory molecule. Th cells thus co-stimulated become more permissive to HIV infection. HIV Env is the key determinant that regulates virus transmission to Th cells and cellular activation of target Th cells. HIV transmission varies depending on Env co-receptor tropisms (CCR5 vs CXCR4) and is sensitive to interference by anti-Env Abs. HIV-induced enhancement of Th cell activation also is triggered by Env and suppressed by anti-Env Abs. On the basis of these findings, we propose to further investigate the HIV Env determinants influencing the efficiency of virus transmission from APCs to Th cells. Our hypothesis is that HIV utilizes the intimate cell-cell contact between APCs and Th cells to spread to Th cells and also to enhance Th cell activation, rendering them more permissive for virus replication, through the action of the virus Env. Therefore, anti-Env Abs, by forming immune complexes with HIV virions, may alter APC-Th cell interactions to blunt virus transmission and replication. To test these hypotheses, in Aim 1, we will define HIV Env determinants that are essential for efficient transmission from APCs to Th cells. Monocytes and MDDCs treated with HIV will be tested as APCs in a co- culture system to stimulate Th1 and Th17 cells and transmit viruses with distinct Envs. The Env variables to be evaluated include co-receptor usage, N-glycan sugar composition, and affinity for ?4?7 and mannose-binding receptors. Aim 2 is to evaluate the ability of anti-Env Abs to retard virus transmission from APCs to Th cells. We will test neutralizing and nonneutralizing monoclonal Abs specific for distinct epitopes, and polyclonal Abs generated by Env epitope-targeted vaccines. Fc-mediated activities of the Abs will also be assessed. Finally, in Aim 3, we will test HIV transmission from APCs in vivo, using the humanized mouse model. Anti-Env Abs with inhibitory activity in vitro will also be evaluated in the mouse model. The proposed experiments will generate data to inform future development of more effective prophylactic agents against HIV.